When designing a mask for a semiconductor integrated circuit, a mask pattern is calculated based on circuit pattern data and optical proximity correction (OPC) processing, and the mask pattern is verified using a lithography compliance check (LCC), etc. This allows detecting critical points of mask pattern data, that is, detecting portions where resist pattern formation defects are highly likely to occur when performing exposure using a mask that is manufacture based on the mask pattern data. The circuit pattern data or OPC conditions are then revised.